Conventionally, in the photographing of X-rays for medicine and industry, a method using a film and a screen was used. In this case, it was inefficient in terms of cost and time due to problems in the development and storage of photographed films.
In order to improve the problem, a digital detector is now widely used. A type of detector can be divided into an indirect conversion method and a direct conversion method.
In the indirect conversion method, X-rays are converted into visible rays, which are converted into electrical signals using a scintillator. In contrast, in the direct conversion method, X-rays are directly converted into an electrical signal using a photoconductive layer. Such a direct conversion method is characterized in that it is suitable for a high resolution system because a separate scintillator does not need to be formed and a light spreading phenomenon is not occurred.
The photoconductive layer used in the direct conversion method is formed by depositing a polycrystalline semiconductor material, such as CdTe, on a surface of a CMOS substrate using a method, such as vacuum thermal deposition.
Meanwhile, a lower electrode and an upper electrode are formed under and over the photoconductive layer, respectively. Charges generated by the photoconductive layer while X-rays are radiated are collected by the lower electrode. To this end, a bias voltage is applied to the upper electrode.
However, even after the radiation of the X-rays is terminated, electrons and holes trapped in the photoconductive layer during the radiation of the X-rays are detrapped, thereby generating a lag signal. In order to reduce such a lag signal, a method for reducing a trap in the photoconductive layer may be suggested, but it is impossible to reduce the trap to a specific level or less due to the nature of the polycrystalline semiconductor formed by vacuum thermal deposition.
Meanwhile, as another method, there may be suggested a method that detrapped charges can be under recombination process by applying a reverse bias voltage to the upper electrode after the radiation of the X-rays. In this case, however, a high voltage needs to be applied in a fast frequency. This has a technical limit. Furthermore, a Schottky diode between the lower electrode and the photoconductive layer and the CMOS substrate may be greatly damaged. Accordingly, the corresponding method cannot be applied.